Sigma Surgical Pvt Ltd offers Cortical Screw. Sigma Surgical Pvt Ltd is an ISO 9001: 2000 certified company. The Cortical Screw [3.5mm, 20tpi] comes in length 10.0 mm to 44.0 mm, with 2 mm length variation. Variation also comes as 3.5mm Cortical screw, 20 tpi. These screws are used in replacement surgery in hospitals and nursing homes.